Studies on the structure and replication of yeast (Saccharomyces cerevisiae) chromosomal DNA will be continued: (1) Mapping replication initiation sites. Partially replicated molecules will be examined in the electron microscope. By studying molecules belonging to a limited size-class it should be possible to determine how specifically sites along a particular molecule are chosen for initiation. (2) Size of the smallest chromosomal DNA molecules. This will be more carefully defined by electron microscopic length measurements, and possibly by electrophoresis and renaturation kinetics. (3) Isolation of a specific chromosomal DNA. Attempts will be made to isolate the DNA corresponding to Chromosome I which can be assayed because it carries a portion of the ribosomal RNA cistrons. (4) Variation in the length of the S-period. By whole cell autoradiography the length of the DNA synthetic period will be measured in yeast with different cell cycle lengths to determine whether the S period is a constant or variable proportion of the cell cycle.